1. Field of the Invention
The present invention relates to an actuator that converts a linear motion into a rotary motion to enable operation of a rotary valve, such as a ball valve, a butterfly valve, etc. and particularly to a spring-return, pneumatic actuator capable of preventing intrusion of the ambient air.
2. Description of the Prior Art
For example, Japanese Patent No. 3738431 discloses a spring return actuator, in which compressed air is supplied to produce a forth motion of a piston. The forth motion is converted into a rotary motion of a rotary shaft with a conversion mechanism having a piston rod, and the snap action of a spring is utilized to produce a back motion of the piston rod, thereby making the rotary shaft counter-rotating.
The conventional actuator has an integral structure comprising a cylinder chamber, a spring chamber and a conversion portion that is a housing for connecting the two chambers. The cylinder chamber is provided therein with a piston and a piston rod joined to the piston. Each of the primary chamber (for the sake of convenience, called a cylinder right chamber) and the secondary chamber (for convenience sake, called a cylinder left chamber) of the piston in the cylinder chamber is equipped with an air supply and exhaust vent for introducing air into the inside of the cylinder.
To the air supply and exhaust vent of the cylinder right chamber is connected a compressed air supply source, such as a compressor, from which compressed air is supplied into the cylinder right chamber to operate the piston. On the other hand, the air supply and exhaust vent of the cylinder left chamber is adapted to discharge the air therein when compressed air has been supplied into the cylinder right chamber and to suck the ambient air in the cylinder left chamber when the spring in the spring chamber has been snapped back.
When the compressed air is supplied into the cylinder right chamber during the course of the actuator operation, an output shaft within the housing is rotated with the movement of the piston and piston rod. At this time, since the air in the cylinder left chamber is discharged to the exterior via the air supply and exhaust vent, the pressure in the cylinder is prevented from increasing to a positive pressure, thereby enabling the piston rod to be operated. On this occasion, the spring is pushed by a spring retainer with the movement of the piston rod, thereby being compressed.
When the supply of the compressed air into the cylinder right chamber is then interrupted to elongate the spring by means of its snap action, the piston rod is operated in the direction opposite to that at the time the compressed air is supplied. As a result, the output shaft is rotated in the direction opposite to that at the time of the operation. At this time, the compressed air in the cylinder right chamber is discharged, whereas the external atmosphere is sucked in the cylinder left chamber to prevent the cylinder left chamber from decreasing to a negative pressure, thereby enabling the piston rod to be operated.
Thus, the conventional actuator of Japanese Patent No. 3738431 is intended to prevent the pressure in the cylinder from being varied through positive supply and discharge of the air to keep the output shaft torque substantially constant and, in particular, to suck the external atmosphere in the cylinder via the air supply and exhaust vent at the time of the spring snap action, thereby preventing the cylinder interior from being brought to a vacuum state. In addition, the conventional actuator is provided with a vent port communicating with the spring chamber so as not to fluctuate the pressure in the spring chamber due to a large variation in internal volume of the spring chamber made at the time of the spring compression and elongation, thereby enabling the air in the spring chamber to be positively supplied or discharged via the vent port.
JP-A 2007-162890 discloses an actuator for a valve, in which a circulating current pipe disposed outside a cylinder is connected to an air supply and exhaust vent provided in the cylinder, and a pressurized air source and a check valve are provided in the circulating current pipe, whereby the ambient air is prevented from being sucked in the cylinder.
However, since the conventional actuator of Japanese Patent No. 3738431 adopts the configuration in which during the course of the back motion, the ambient air is sucked in the cylinder left chamber via the air supply and exhaust vent communicating with the external atmosphere, there is a possibility of moisture, such as seawater, rainwater, etc. and foreign material, such as dust, etc. invading the cylinder interior as being mixed with the ambient air.
For this reason, when the conventional actuator is used outdoors, there is a case where the piston cannot smoothly be operated due to the moisture and foreign matter. Particularly, when plural actuators of this type are used for a plant in a coastal neighborhood, these possibly suck therein the ambient air containing seawater and saline matter, resulting in that the cylinder interior would be prone to rusting and exhibit a high attrition rate. Furthermore, even when the spring chamber is provided therein with a vent port, similarly to the case of the cylinder, invasion of moisture, such as seawater, and foreign matter brings about a high attrition rate. Moreover, the formation of the vent port would make the contour of the actuator larger, i.e. difficult to attain compactness of the actuator, and increase the number of man-hours and parts for the actuator manufacture. These are problematic.
The invasion of moisture or foreign matter into the main body of the actuator prevents smooth reciprocation of the piston, resulting in failure to play the role as an emergency shutoff valve. While water, dust, etc. are easy to enter the actuator when the air supply and exhaust vents open upward, particularly when the direction in which the air supply and exhaust vents open is charged, the directionality of the main body of the actuator to be installed is restricted, causing a problem of limiting the places of installment of the actuator main body.
In order to prevent invasion of moisture, foreign matter, etc. it is conceivable that the inside of the actuator is subjected to coating or that the main body of the actuator is formed of an antirust material. In either case, however, a problem will arise in which the manufacturing cost is increased. On the other hand, even when the inlet of the air supply and exhaust vent is provided with a filter etc., there is a possibility of fine foreign matter passing through the filter and entering the main body of the actuator to induce an interference with the operation or of use of the filter over a long period of time inducing clogging with the dust accumulated.
In view of the above, it is conceivable that the actuator is formed into a sealed structure so as not to be exposed to the ambient air. In this case, however, the cylinder right chamber is possibly brought to a negative pressure state during the course of the back motion of the piston to pull the piston back, thereby making it difficult to normally operate the output shaft. What is conceivable in order to eliminate this problem is to adopt a structure in which the diameter of the cylinder is made larger to increase the internal volume of the cylinder right chamber, thereby suppressing a negative pressure state otherwise produced during the operation of the piston. According to this structure, however, a problem will be posed, in which the amount of the compressed air flowing into the conversion portion consumed will be increased.
On the other hand, since JP 2007-162890 requires the circulating current pipe to be disposed outside the cylinder, it fails to satisfy the demand for making a valve actuator compact.
As different means for increasing the internal volume of the cylinder, a structure in which the axial length of the cylinder right chamber is extended is conceivable. According to this structure, however, since the cylinder elongates excessively in the lengthwise direction, desired compactness of the actuator main body will not be satisfied and the installation space will be increased. These are also problematic.
The present invention has been accomplished in order to solve the conventional problems and has as its object to provide a compact, spring-return, pneumatic actuator capable of preventing invasion of extraneous material, such as moisture, dust, etc. into the actuator main body to smoothly operate in order and also capable of suppressing a variation in pressure in the main body to smoothly operate with a prescribed torque.